change assertResultFailed(f : =>, code: Int) to return ErrorResult

In standard-usage-schema, I would like to be able to check substring for the 400 message. The reason is, the exact error messages for error 400 returned by Xerces and Saxon are different. I don't want to have to conditionally code this in the standard-usage-schema tests. Instead, I want to be able to check for substring like this:

```
  test("Slice Action without serviceCode should generate 400 with the appropriate response message") {
    val req = request("POST", "/servers/events", "application/atom+xml", sliceActionWithoutServiceCode)
    val validatorResult = this.assertResultFailed(atomValidator.validate(req, response, chain), 400)
    assert(validatorResult.message.contains("serviceCode"))
    //assertResultFailed(atomValidator.validate(req, response, chain), 400, "Bad Content: Required attribute @serviceCode is missing")
  }
```